Is anything my employer is doing, detailed below, actually illegal or am I just stuck with no recourse for their behavior?

At my workplace, I am the supervisor and manager for the factory's overnight 3rd shift. If I need to take a day off for any reason be it illness, injury, or a flare up of one of my military related disabilities (that my employer is fully aware of and has multiple notes from my doctors detailing their impact and chronic status), I am simply told "no, come in anyway". If I insist the absence cannot be avoided I'm then told they won't (as opposed to can't) get anyone to cover for me and if I don't come in the entire shift is cancelled and no one will be paid for the missed hours. Not wanting to harm co-workers like that I have worked with 103+ degree fevers, while partially blind (ocular migraines), with severe sciatic nerve inflammation, and even partially dislocated joints. Even giving several weeks of prior notice for upcoming surgical procedure absences result in the same 'threats', and I've had to reschedule or cancel surgeries as a result.